1. Field of the Invention
The invention relates to photolithographic exposure, and in particular to a method of nonlinear photolithographic exposure.
2. Description of the Related Art
The half-pitch resolution limit of a photolithographic system is given by 0.25λ/NA, where λ is the exposure wavelength, and NA is the numerical aperture (sine of the maximum angle in the final lens). Traditionally, resolution has been improved by reducing the wavelength. For example, the industry has shifted from predominant use of KrF (248 nm) to ArF (193 nm) lasers as light sources. A shift to extreme-ultraviolet sources using 13.5 nm wavelength is popularly being contemplated. In addition, electron beams offer even shorter wavelength (˜4 μm). However, the use of short wavelength radiation generates secondary electrons if the energy of a single radiation quantum exceeds the ionization potential of the exposed material. The secondary electrons constitute an additional exposing agent, and degrade the resolution of the lithography process. Resolution below 40 nm becomes less repeatable due to the stochastic behavior of the secondary electrons. Even more significantly, the secondary electrons contribute significantly to line-edge roughness, especially in chemically amplified resists.
Low-energy massive particles such as electrons or atoms can constitute short-wavelength, non-ionizing radiation. The de-Broglie relation gives the particle wavelength as λ=h/(mv), where h is Planck's constant, m is the particle mass, and v is the particle velocity. However, given their low kinetic energy, it is difficult to build optical systems that direct their motion. In addition, upon impacting the sample, their motion becomes completely uncontrollable. In the case of electrons, charging and, scattering can cause spreading and subsequent loss of resolution. In the case of atoms, van der Waals forces, thermal motion, surface diffusion or etching can cause loss of feature resolution and smoothness as well as feature damage.
Current state-of-the-art optical lithography systems utilize an ArF excimer laser source, and an imaging medium of water. The water immersion medium reduces the wavelength from 193 nm to 134 nm. Consequently, the minimum half-pitch resolution achievable using conventional resolution enhancement is about 36 nm. In order to use the same optical system to exceed the optically defined half-pitch limit, a second exposure of a second coated photoresist layer is normally required on the same tool. This will result in reduced throughput, higher consumption of costly materials, and reduced yield due to overlay errors.
Most state-of-the-art photoresists are characterized by the absence of photo-bleaching. In other words, the absorption is independent of intensity. As a result, the photoresist half-pitch image resolution matches the optical prediction.
It is possible for photoresist resolution to exceed the optical resolution by as much as a factor of two, if special absorption mechanisms are used. One such mechanism is two-photon absorption, which is the simultaneous absorption of two photons as effectively one photon with effectively half the wavelength. The absorption is a second-order function of the intensity. This enables significant contrast enhancement. Furthermore with an appropriate exposure strategy, such as nonlinear multiple exposure (NOLMEX), the resolution can be enhanced as well. Two-photon absorption suffers from three drawbacks, however. First, the two-photon absorption is only allowed when single-photon absorption is not. This constrains the required chemistry. Second, the absorption is proportional to the square of the intensity, so the drop in intensity with loss of focus means the focus window of the image formed in the photoresist is reduced. Third, two-photon absorption leads to ionization which provides no advantage over merely reducing the wavelength.
It is not necessary to rely on two-photon absorption to achieve nonlinear absorption. For example, photobleaching is a well-modeled effect.
In U.S. Pat. No. 5,739,898 “Exposure method and apparatus”, a photosensitive material is utilized in which “effective light intensity” is nonlinear with respect to intensity. In other words, the latent image is proportional to a nonlinear function of intensity. By use of multiple exposures mutually shifted with respect to one another, a higher resolution pattern is formed. The dependence on multiple exposures obviously leads to reduced throughput, higher cost, and greater sensitivity to alignment error. The invention requires only a single exposure. Double or multiple exposures on the same tool are also used in JP05082407A2 “Method for forming fine pattern,” U.S. Pat. No. 6,245,492 “Photoresist system and process for aerial image enhancement,” and U.S. Pat. No. 5,407,785 “Method for generating dense lines on a semiconductor wafer using phase-shifting and multiple exposures.”
Likewise in JP10326746A2 “Method of Forming Mask Pattern,” a photo-bleaching film is used on top of photoresist, to improve resist shape and control. However no anti-bleaching is used, so no pitch resolution enhancement can be realized.
In U.S. Pat. No. 7,022,452 “Contrast enhanced photolithography”, a photobleaching layer is used as a contrast enhancing layer disposed on top of an imaging photoresist layer. While contrast improvement is demonstrated, there is no resolution enhancement in merely adding a contrast enhancing material on top of photoresist. The invention offers a two-fold resolution enhancement.
In JP62135821A2 “Formation of Pattern” a thin bleaching film is used to expose photosensitive resin, but the result is to improve the uniformity of contrast enhancement, not resolution.
In JP05158244A2 “Pattern Forming Method” an anti-bleaching negative photoresist is itself developed as the imaging layer. However, no bleaching occurs, so the method cannot offer higher resolution, since the top portion of the resist darkens first, allowing light to spread as it propagates to the bottom of the resist.